drm/i915: constify hotplug function vtable.

Use a macro to avoid mistakes, this type of macro is only used
in a couple of places.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/af229c1e8ec894f177b344ab77b90e32c33428de.1632869550.git.jani.nikula@intel.com